Buff at the cost of your lifespan sounds like big mom's devil fruit power on steroids since Imu can grant it to just anyone. BM was shown to do it only to herself during her flight in Wano. Possessing someone with soul sounds like Moria's. Only this time, Imu give a portion of his own soul instead of stealing and transferring random shadow to dead bodies. Such recipe for OP devil fruit. Both time, Brook was highlighted to know how to counter it. Both time without haki. Now is the same. He'll lead the way so weaker ShPs can still be relevant combat wise. Just like them being relevant in fighting Oars despite the huge difference in power and oars being semi immortal